The objectives of this research are to investigate (1) effects of ionizing radiation on the immature spinal cord, (2) the reparative and regenerative capacities of the immature nervous system, and (3) the manner in which ionizing radiation alters these capacities. All experiments are carried out in rats because the developmental state of these animals at birth makes it possible to study the immature nervous system ex utero. Neurologic, histopathologic and autoradiographic methods are used in this research. The responses of the immature nervous system to injury are compared and contrasted with those reported by others in the adult nervous system; this, perhaps, will provide some insight into the reactions and limitations occurring in the mature state. In addition to the experimental data, the information derived from intact, control animals will contribute to the understanding of normal maturation of the nervous system.